MODIFICATION: Perform at your own risk...
This modification will disable the valve in the exhaust (at the exit of the cat) such that it stays open 100% of the time, instead of closing at low RPMs. The end result is more Rumble at low RPMs... and it sounds goooood! This would work whether or not you're still using the stock can or if you go to an aftermarket slip-on.
There is a flapper valve on the exit pipe of the Cat right before the slip-on. This particular valve is there in order to pass Noise level requirements from the 0-5000-ish RPMs. This is similar to other bikes - I know my '08 CBR1000RR was the same way.
At the valve, there is a plastic cover. On the side facing you, there is a plastic clip, remove the plastic retaining clip with a screwdriver (removes very easily). Now the plastic cap will come right off. Once off, you will see the two cables that operate the flapper valve (normally closed at idle). Loosen the nut for each cable where they attach to the bracket/valve. Once loose, slide the cable out of its location and release the cable's end from the valve. The next step is easier if you remove the tank side plastics (also requires the top tank plastic to be removed and the seat). There is servo that operates the cables. It's locate under the seat, but you access the cables from below the tail (above the hugger). Remove the cables from the servo with some needle nose pliers or what ever tool you prefer. With a long screw driver from the right side, I was able to pry the cable from it's mount (just to the right of the servo), there are two other clips holding the cables in place, remove the cables from there and then remove the cables. Once this has been done put the cap back on the valve and carefully slide the retaining clip back in. The valve will stay in the 100% open position full time due to it being "spring loaded" to do so. No codes are thrown since the servo is still plugged in and operating normally. Reassemble the plastics, you're done.
Now you got more RUMBLE
EDIT: Per RocketJockey's suggestion, I have now safety wired the valve in the open position. Although it is a strong spring, he heard of someone at the track that only had a few laps to go and went into limp mode because the spring went out on them. Better safe than sorry - so I wired it open.
Picture 1: Valve that is on the exit side of the CAT. I'm pointing at the clip that holds the cover on.
Picture 2: Clip is shown partially removed, when you reinstall you will want to wiggle the cover a little to ensure the clip seated the cover properly (don't want to loose it going down the road).
Picture 3: Valve with the cover off.
Picture 4: Shows where the other end of the cables attach (to the servo motor). This is looking in from the right side of the bike w/the tank plastic removed.